Production of instant soymilk powders by ultrafiltration, spraydrying and fluidized bed agglomeration

Nakarin Jinapong, Manop Suphantharika *, Pimon Jamnong

Department of Biotechnology, Faculty of Science, Mahidol University, Rama 6 Road, Bangkok 10400, Thailand

Abstract

Production of instant soymilk powders was completed in three stages – ultrafiltration, spray drying, and fluidized bed agglomeration.Ultrafiltration increased total solids, protein, and fat contents, but decreased carbohydrate and ash contents of soymilk, leading to anincrease in particle size, wettability, and dispersibility of the resultant spray-dried powders. However, all the spray-dried soymilk powderswere very small (<25 lm) and very cohesive leading to their poor flowability. Agglomeration of the spray-dried powders with maltodex-trin as an aqueous binder solution using a fluidized bed agglomerator improved the handling and reconstitution properties of the pow-ders. The optimum binder concentration was found to be 10% w/v maltodextrin which resulted in the largest particle size of theagglomerated powder (260 lm) having a good flowability and low cohesiveness. The wettability of this agglomerated powder (wettingtime = 42 s) was good but its dispersibility (61%) could be improved.

Keywords: Soymilk; Spray drying; Agglomeration; Ultrafiltration; Fluidized bed

1. Introduction

Soymilk is becoming more and more popular as con-sumers become more health conscious and seek out alter-natives to dairy products. Basically, soymilk is a waterextract of soybeans, closely resembling dairy milk inappearance and composition. It contains high amounts ofprotein, iron, unsaturated fatty acids, and niacin, but lowamounts of fat, carbohydrates, and calcium as comparedwith cow’s milk and human milk (Liu, 1997). Soymilk isalso touted as a healthy food because it is cholesterol andlactose free and contains phytochemicals. It is recom-mended for those who are allergic to milk protein or havelactose intolerance and those who have special health orreligious diet requirements (Liu, 1997; Pomeranz, 1991).

Ultrafiltration is a hydraulic pressure-operated mem-brane technique used as a concentration step in the foodindustry, including soymilk processing. Its use is an attrac-

* Corresponding author. Tel.: +66 2 201 5314; fax: +66 2 354 7160.E-mail address: scmsp@mahidol.ac.th (M. Suphantharika).

tive alternative to conventional evaporation processes as itis a non-thermal treatment, as well as one with mild oper-ating conditions. The applications of ultrafiltration in soy-milk processing are not only concentration, but alsoremoval of anti-nutritional low molecular weight compo-nents such as oligosaccharides and phytic acid (Ang, Kwik,Lee, & Theng, 1986; Omosaiye, Cheryan, & Matthews,1978). The low molecular weight oligosaccharides, raffinoseand stachyose have been implicated as causative factors indigestive disturbances, such as flatulence while phytic acidhas been considered to lower the mineral biodegradabilityby forming insoluble complexes with di- and tri-valent ionsat neutral pH (Ang et al., 1986; Omosaiye et al., 1978;Pomeranz, 1991).

Spray drying is the most widely used commercialmethod for drying milks because the very short time of heatcontact and the high rate of evaporation give a high qualityproduct with relatively low cost (Liu, 1997). A dry powderproduct is highly desirable since it not only possesses longshelf-life, but also requires relatively low transportationcost and storage capacity and the product can be distrib-

uted over a wide area. Thus, a process for producing adried soymilk powder that is soluble and without loss ofnutritive value is highly desirable. However, relatively littleresearch, especially recent studies have been carried out ondehydration of soymilk. Spray drying has been introducedfor production of dehydrated soymilk powders by Wijer-atne (1993). A spray-drying system for soymilk has beencharacterized at various combinations of inlet air tempera-ture, feed rate, and atomizer speed on outlet air and prod-uct temperatures, thermal and evaporative efficiencies(Perez-Munoz & Flores, 1997a), and particle size ofspray-dried soymilk powders (Perez-Munoz & Flores,1997b). Spray-drying studies were also carried out on theultrafiltered soymilk concentrates (Ang et al., 1986). Theresults revealed that the nitrogen solubility index (NSI) ofthe spray-dried powder improved with percent waterremoval during ultrafiltration and also by the addition ofsucrose to the concentrate prior to spray drying. However,the detailed characteristics in terms of physicochemical,physical, morphological, and reconstitution properties ofthe spray-dried soymilk powders have not yet been evalu-ated by these previous workers.

Agglomeration can be defined as the size enlargementprocess in which the starting material is fine particles suchas dusts or powders join or bind with one another, resultingin an aggregate porous structure much larger in size thanthe original material, such that the primary particles canstill be identified (Ortega-Rivas, 2005; Parikh, Bonck, &Mogavero, 1997; Schuchmann, 1995). The granules areheld together with bonds formed by the binder used toagglomerate. Agglomeration is used mainly to improveproperties related to handling and reconstitution of pow-ders. In the food industry, agglomeration is used for instantproducts such as coffee, milk, or cocoa that disperse and/ordissolve quickly in liquids like water or milk (Caric, 2003;Schubert, 1993; Schuchmann, 1995). The reconstitutionproperties such as wettability, sinkability, dispersibility,and solubility are enhanced. These properties depend onthe type of agglomeration process and on the operatingconditions during agglomeration. Fluidized bed agglomer-ation is one of the most suitable processes leading toagglomerates with high porosity and good mechanicalresistance for handling and packaging (Turchiuli, Eloualia,El Mansouri, & Dumoulin, 2005). This process generallyworks by spraying binder liquid onto a bed of fluidizedparticles, where upon wetting, the particles will be boundtogether by liquid bridges. Depending on the type of binderused, the liquid bridge will either solidify with cooling ordry with heating to form a solid bridge (Parikh et al.,1997; Tan, Salman, & Hounslow, 2006). The spray-driedpowders obtained from small scale spray dryers often havea small particle size, <50 lm, with poor handling andreconstitution properties (Masters, 1991). These powdersrequire agglomeration in order to improve handling andreconstitution properties (Fuchs et al., 2006; Turchiuliet al., 2005). Wijeratne (1993) suggested that agglomerationwill improve the wettability of the spray-dried soymilk

powder. At present, however, there is no report on theagglomeration of the spray-dried soymilk powders for pro-ducing instant soymilk products.

The objective of this study was to investigate the feasibil-ity of production of instant soymilk powders. The produc-tion was completed in three stages – ultrafiltration, spraydrying, and fluidized bed agglomeration. Influences of soy-milk concentration and binder concentration on spray dry-ing and agglomeration, respectively, were evaluated.Powders were characterized before and after agglomerationin terms of physicochemical, physical, morphological, andreconstitution properties.

2. Materials and methods

2.1. Materials

Soymilk (5.2% w/w total solids content) was providedby Green Spot (Thailand) Ltd., Pathum Thani, Thailand.Soymilk was produced as a water extract of ground,soaked soybeans heated at 70–80 �C for 10–15 min andthen cooled to room temperature. Maltodextrin (D-Perse3�) of dextrose equivalent (DE) value 14 was supplied bySiam Modified Starch Co., Pathum Thani, Thailand.

2.2. Ultrafiltration

Soymilk (30 l) was concentrated on a Nitto Ultrafiltra-tion Tubular Module (NTU-2020-P18A, Nitto ElectricIndustrial Co., Ltd., Osaka, Japan), which used 18 tubes(O.D. = 16 mm, I.D. = 11.5 mm, length = 2619 mm) oftubular membranes, giving an effective membrane surfacearea of 1.6 m2. A hydrophilic polyolefin membrane of20,000 nominal molecular weight cut-off was used in allexperiments. A series of experiments were carried out attemperature of not greater than 40 �C and the soymilks wereconcentrated to different solids contents, i.e. 7.0%, 9.9%,13.0%, 15.6%, and 20.7% w/w. During each run the concen-trate was allowed to recirculate back to the feed tank.

2.3. Spray drying

Soymilk and concentrated soymilks were fed into aspray dryer (Mobile Minor Spray Dryer, Niro A/S, Soe-borg, Denmark) by a peristaltic pump, and atomized tosmall droplets by a centrifugal vaned atomizer wheel witha rotational speed of 23,000 rpm (4 bar air pressure) in aco-current air flow system. The inlet air temperature wasset at 180 ± 1 �C and the outlet air temperature was keptat 80 ± 1 �C by varying the feed rate in the range of 33–51 ml/min. Finally, the powder was then collected from acyclone and a sample was taken for further analysis.

2.4. Fluidized bed agglomeration

Agglomeration of the spray-dried soymilk powders wasperformed in a top-sprayed fluidized bed granulator and

dryer (Strea-1, Fluid Bed Laboratory, Aeromatic-FielderAG, Bubendorf, Switzerland) with 2 l vessel capacity. Inthis study, aqueous solutions of maltrodextrin DE 14 ofvarious concentrations, i.e. 0%, 5%, 10%, 15%, and 20%w/v, were used as a binder. The spray-dried powder weigh-ing 200 g was placed in the product container, and fluidizedby means of an upward flowing air stream. The tempera-ture of the inlet fluidizing air entering the bed was set at50 �C. The binder solution (200 ml) was fed by a peristalticpump at a flow rate of 11–12 ml/min to a two-fluid spraynozzle where the binder was sprayed onto the fluidizedbed of soymilk powder. The air pressure on the nozzlewas 1.5 bar. During agglomeration, it was necessary to reg-ularly increase the fluidizing air flow to maintain a goodfluidization of the enlarged agglomerates. When the bindersolution was consumed, the product was dried for 15 minat a temperature of 50 �C.

2.5. Methods of measurement

2.5.1. Viscosity measurement

Viscosity of soymilk and concentrated soymilks wasdetermined by a Programmable Viscometer (BrookfieldDV-II+, Brookfield Engineering Laboratories, Inc., Mid-dleboro, MA) with a small sample adapter. The samples(8 ml) were measured by using a spindle No. 18 at an oper-ating shear rate of 105.6 s�1 and a temperature of 25 �C.

2.5.2. Proximate analysis

Proximate analyses for moisture, ash, and protein con-tents of the spray-dried soymilk and agglomerated soymilkpowders were carried out using AOAC methods (AOAC,2000). Fat content was determined by the modified Mojon-nier method (AOAC, 2000). Total carbohydrates contentwas determined by subtracting the ash, protein, and fatpercentages from 100%. All tests were carried out in tripli-cate and the mean calculated.

2.5.3. Particle size measurement

For the spray-dried soymilk powders, the particle sizedistribution, and volume-weighted mean diameter weremeasured by the dry method in a laser diffraction particlesize analyzer (Mastersizer 2000; Malvern InstrumentsLtd., Worcestshire, UK) fitted with a Scirocco 2000 drypowder feeder unit. The volume-weighted mean diameter(d4,3) was calculated as follows:

d4;3 ¼P

nid4iP

nid3i

ð1Þ

where ni is the number of particles of diameter di. The par-ticle size distribution of the powder was measured as thespan which is defined as

span ¼ d90 � d10

d50

ð2Þ

where d90, d10, and d50 are the equivalent volume diametersat 90%, 10%, and 50% cumulative volume, respectively.

For the agglomerated soymilk powders, due to theirmuch larger particle sizes, sieve analysis using a vibratorysieve shaker (Retsch GmbH and Co., Haan, Germany)with a series of seven sieves was used to determine theirparticle size distributions. The aperture sizes of sieves were75, 90, 125, 150, 180, 250, and 300 lm. The agglomeratedsoymilk powder (50 g) was put on the sieve’s series and sha-ken at 60 Hz for 30 min. The size distribution wasdescribed by log-normal distribution relationship. Fromthe log-probability plots, mass-weighted geometric meandiameter as well as geometric standard deviation definedby the slope of the log-normal curve were determined.

2.5.4. Morphological studyThe appearance, size, and shape of the powder samples

were investigated by placing the powders on aluminum stubsusing a double-sided adhesive tape. The samples were thencoated with gold and were examined with a scanning electronmicroscope (SEM S-2500, Hitachi Science Systems, Ibaraki,Japan) operating at 15 kV accelerating voltage.

2.5.5. Bulk and tapped densitiesPowder was gently loaded into a 100 ml tared graduated

cylinder to the 100 ml mark and weighed. The volume readdirectly from the cylinder was then used to calculate thebulk density (qbulk) according to the relationship: mass/vol-ume. For the tapped density (qtapped), the cylinder wastapped 1250 times, using a VanKel Tapped Density Tester(ASTM Version, Varian, Inc., Cary, NC) with a displace-ment amplitude of 3 ± 0.3 mm. The volume of the samplewas then read and used in the calculation. The results werecalculated from three replicate measurements.

2.5.6. Particle density

Particle density (qparticle) of the powder sample was ana-lyzed according to A/S Niro Atomizer (1978c) with somemodifications. The powder sample (1 g) was transferredinto a 10 ml measuring cylinder with a glass stopper. Then5 ml of petroleum ether was added and the measuring cyl-inder was shaken until all the powder particles were sus-pended. Finally, all the powder particles on the wall ofthe cylinder were rinsed down with a further 1 ml of petro-leum ether (6 ml in total) and the total volume of petro-leum ether with suspended powder was read. The particledensity was calculated as follows:

qparticle¼weight of powder ðgÞ

total volume of petroleum ether with suspended powder ðmlÞ�6

ð3Þ

2.5.7. Porosity

Porosity (e) of the powder samples was calculated usingthe relationship between the tapped (qtapped) and particle(qparticle) densities of the powder as shown below:

e ¼ðqparticle � qtappedÞ

qparticle

� 100 ð4Þ

2.5.8. Flowability and cohesiveness

Flowability and cohesiveness of the powder were evalu-ated in terms of Carr index (CI) (Carr, 1965) and Hausnerratio (HR) (Hausner, 1967), respectively. Both CI and HRwere calculated from the bulk (qbulk) and tapped (qtapped)densities of the powder as shown below:

CI ¼ðqtapped � qbulkÞ

qtapped

� 100 ð5Þ

HR ¼qtapped

qbulk

ð6Þ

Classification of the flowability and cohesiveness of thepowder based on the CI and HR values are presented inTables 1 and 2, respectively.

2.5.9. Wettability

Wettability of the powder sample was determinedaccording to A/S Niro Atomizer (1978a) with some modi-fications. An amount of distilled water (100 ml) at25 ± 1 �C was poured into a 250 ml beaker. A glass funnelheld on a ring stand was set over the beaker with the heightbetween the bottom of the funnel and the water surface of10 cm. A test tube was placed inside the funnel to block thelower opening of the funnel. The powder sample (0.1 g)was placed around the test tube and then the tube waslifted while the stop watch was started at the same time.Finally, the time was recorded for the powder to becomecompletely wetted (visually assessed as when all the powderparticles penetrated the surface of the water).

2.5.10. Dispersibility

Dispersibility measurement was performed according tothe procedure described in A/S Niro Atomizer (1978b) withsome modifications. Distilled water (10 ml), at 25 ± 1 �C,was poured into a 50 ml beaker. The powder (1 g) was addedinto the beaker. The stop watch was started and the samplewas stirred vigorously with a spoon for 15 s making 25 com-plete movements back and forth across the whole diameterof the beaker. The reconstituted soymilk was pouredthrough a sieve (212 lm). The sieved soymilk (1 ml) was

Table 1Classification of powder flowability based on Carr index (CI)

CI (%) Flowability

<15 Very good15–20 Good20–35 Fair35–45 Bad>45 Very bad

Table 2Classification of powder cohesiveness based on Hausner ratio (HR)

HR Cohesiveness

<1.2 Low1.2–1.4 Intermediate>1.4 High

transferred to a weighed and dried aluminum pan and driedfor 4 h in a hot air oven at 105 ± 1 �C. The dispersibility ofthe powder was calculated as follows:

% dispersibility ¼ ð10þ aÞ �% TS

a� 100�b100

ð7Þ

where a = amount of powder (g) being used, b = moisturecontent in the powder, and % TS = dry matter in percent-age in the reconstituted soymilk after it has been passedthrough the sieve.

2.6. Statistical analysis

All measurements were made in triplicate for each sam-ple. Results are expressed as mean ± standard deviations.A one-way analysis of variance (ANOVA) and Tukey’s test(p 6 0.05) were used to establish the significance of differ-ences among the mean values of the physicochemical, phys-ical, and reconstitution properties of the spray-dried andagglomerated soymilk powders. The data were analyzedusing SPSS version 12.0 Windows program (SPSS Inc.,Chicago, IL).

3. Results and discussion

3.1. Ultrafiltration of soymilk

Multiple-effect evaporators are commonly used in thefood industry to concentrate dilute solutions prior to spraydrying because it is generally less expensive to removewater by evaporation rather than using a spray dryer.However, evaporation might cause thermal degradationof food components and loss of volatiles like flavors. Ultra-filtration could overcome these disadvantages. The mainadvantages of membrane concentration over evaporationare that (1) the food is not heated, (2) there is a negligibleloss of quality, (3) there is less loss of volatiles, and (4) ituses energy more efficiently because there is no phasechange (Ramaswamy & Marcotte, 2006). In this study, afew batches (40 l each) of soymilk were concentrated usinga double-effect climbing-film evaporator (QVF Process Sys-tem Ltd., Stafford, UK) (data not shown). The maximumsolids content of the resultant soymilk concentrates wasabout 20%. Above this level of concentration the viscosityrapidly increased and the concentrates tended to formcurd. This concentration is comparable to that obtainedby ultrafiltration (see Section 3.3). However, ultrafiltrationwas preferable not only for concentration, but also removalof anti-nutritional low molecular weight components ofsoymilk (Ang et al., 1986; Omosaiye et al., 1978) whichcannot be achieved by evaporation.

3.2. Spray drying of soymilk

Relationships between feed rate of ultrafiltered soymilksduring spray drying, viscosity and their concentrations are

Fig. 3. Particle size distributions of spray-dried soymilk powders pro-duced from soymilk (control) and ultrafiltered soymilk concentrates ofdifferent concentrations.

shown in Fig. 1. As would be expected, in order to controlthe outlet air temperature of 80 ± 1 �C, the feed rate wasincreased with increasing the total solids content of soy-milks. Soymilk viscosity also increased from 2.2 to22.4 mPa s with increasing concentrations from 5.2% to20.7%.

Correlation between the mean particle size of the spray-dried soymilk powders and total solids content of the feedsolutions is shown in Fig. 2. The mean particle sizes of soy-milk powders sharply increased from 14.54 to 23.59 lmwith increasing the solids contents from 5.2% to 13.0%and then increased slightly thereafter. According to Mas-ters (1991), for rotary atomizers, the mean liquid dropletsize varied directly with feed rate and viscosity of the feedliquid at constant atomizer speed. The difference betweenthe mean sizes of wet spray and dried particles showed dif-ferent trends when feed-solids concentration was changedat constant inlet temperatures. For products characterizedby their film-forming properties, there was little differencebetween the mean sizes at high feed concentration. Atlow concentrations, however, the difference was significant.Therefore, the lower the solids content of soymilk, whichresulted in the lower feed rate and viscosity (Fig. 1), the

Fig. 1. Effect of total solids content of soymilk (control) and ultrafilteredsoymilk concentrates on viscosity and feed rate during spray drying ofsoymilks.

Fig. 2. Effect of total solids content of soymilk (control) and ultrafilteredsoymilk concentrates on the mean particle size of spray-dried soymilkpowders.

smaller liquid droplets formed during atomization resultingin much smaller dried particles than those obtained at highsolids content. For a two-fluid nozzle atomizer, Keogh,Murray, and O’ Kennedy (2003) found that the particle sizeof the spray-dried powders increased linearly with the vis-cosity of the ultrafiltered whole milk concentrates.

The particle size distributions for various spray-driedpowders produced from different soymilk concentrationsare shown in Fig. 3. All the powders showed smooth uni-modal size distribution curves of which their peaks shiftedto larger particles as the soymilk concentration increased.This result implies that the solids content of soymilkaffected mainly on the mean particle sizes and to a muchlesser extent on the size distributions of the spray-driedpowders. The scanning electron micrographs of spray-driedsoymilk powders show an increase in particle size withincreasing total solids content of soymilks (Fig. 4) whichis consistent with those determined by the particle size ana-lyzer (Table 4). The particles produced from the low solidssoymilks are more wrinkled than those produced from thehigh solids ones. These differences in the microstructurecould be due to the higher amount of water was removedfrom the low solids liquid droplets during dehydrationresulting in higher shrinkage of the droplets, which tendedto form wrinkles.

3.3. Proximate analyses of spray-dried soymilk powders

Table 3 shows the composition of the different spray-dried soymilk samples. It can be seen that the powdersobtained from the ultrafiltered concentrates exhibited a sig-nificant (p 6 0.05) increase in protein and fat contents, butdecrease in carbohydrate and ash contents as comparedwith that obtained from the non-ultrafiltered soymilk. Thiswas due to the removal of low molecular weight oligosac-charides and inorganic salts by ultrafiltration as describedby Ang et al. (1986) and Omosaiye et al. (1978). Most ofthe soy proteins have molecular weights ranging from200,000 to 600,000 (Ang et al., 1986; Pomeranz, 1991).The use of 20,000 molecular weight cut-off membranes

Fig. 4. Scanning electron micrographs of spray-dried soymilk powders produced from soymilk (control) and ultrafiltered soymilk concentrates of differentconcentrations: (a) 5.24% (control), (b) 9.85%, (c) 12.98%, and (d) 20.69% w/w total solids contents at 1000� magnification.

Table 3Proximate analyses (% w/w, dry basis) of spray-dried soymilk powders produced from soymilk (control) and ultrafiltered soymilk concentrates of differentconcentrations

Soymilk concentrationA Protein Fat Ash Carbohydrate

5.24 ± 0.03f,B 48.79 ± 1.04b 28.77 ± 0.36d 4.97 ± 0.01a 17.47 ± 1.40a

7.04 ± 0.01e 52.34 ± 0.45a 30.43 ± 0.19c 4.39 ± 0.05ab 12.84 ± 0.30bc

9.85 ± 0.01d 51.51 ± 1.09a 33.99 ± 0.33a 3.43 ± 0.01cd 11.06 ± 1.42c

12.98 ± 0.13c 50.98 ± 0.29a 32.12 ± 0.14b 3.80 ± 0.54bc 13.12 ± 0.94bc

15.57 ± 0.01b 51.61 ± 0.29a 31.33 ± 0.20b 3.14 ± 0.07d 13.91 ± 0.16b

20.69 ± 0.27a 50.31 ± 1.03ab 33.54 ± 0.39a 3.48 ± 0.09cd 12.67 ± 0.80bc

Assays were performed in triplicate. Mean ± SD values in the same column with different superscripts are significantly different (p 6 0.05).A Soymilk concentration is expressed in terms of % w/w (wet basis) total solids content.B Control soymilk as received without ultrafiltration.

should retain these proteins. Fat globules are also largerthan the pore size of the membrane and are thus expectedto be retained by the membrane. Ash content was notreduced as much even though salts should freely permeatethrough the membrane. This is probably due to solute–sol-ute interaction between the salts and a rejected species suchas protein as suggested by Ang et al. (1986) and Omosaiyeet al. (1978).

In comparison among the soymilk powders obtainedfrom the ultrafiltered soymilks of different concentrations,the powders exhibited practically no difference in theircompositions. This means that the removal of the lowmolecular weight components occurred mainly at thebeginning of the ultrafiltration experiments. The explana-tion could be due to the presence of high molecular weightsolute or colloid and insoluble matter which resulted in sol-ute accumulation at the membrane surface during ultrafil-

tration and produced a layer of finite, and frequentlylarge, hydraulic flow resistance and thereby decreased theoverall hydraulic permeability of the membrane as thesolution was being concentrated. In this study, it was foundthat concentration to more than approximately 20% totalsolids was impractical due to rapid increases in viscosityand pressures.

3.4. Physical, handling, and reconstitution properties of

spray-dried soymilk powders

Physical, handling, and reconstitution properties of thedifferent spray-dried soymilk samples are presented inTables 4–6, respectively. All physical properties of the soy-milk powders seemed to be unaffected by the total solidscontent of soymilks from which the powders were pro-duced except for the mean particle sizes which increased

Table 4Physical properties of spray-dried soymilk powders produced from soymilk (control) and ultrafiltered soymilk concentrates of different concentrations

Soymilk concentrationA Moisture (%) Particle size (lm) Span qbulk (g/cm3) qtapped (g/cm3) qparticle (g/cm3) Porosity (%)

5.24 ± 0.03f,B 4.45 ± 0.11b 14.54 ± 0.16f 1.34 ± 0.05b 0.21 ± 0.00a 0.35 ± 0.00a 1.25 ± 0.00ab 72.00 ± 0.00ab

7.04 ± 0.01e 3.81 ± 0.16c 14.92 ± 0.14e 1.26 ± 0.00bc 0.22 ± 0.01a 0.32 ± 0.01bcd 1.25 ± 0.00ab 74.13 ± 0.46a

9.85 ± 0.01d 3.29 ± 0.17c 19.00 ± 0.09d 1.48 ± 0.04a 0.21 ± 0.00a 0.31 ± 0.00d 1.04 ± 0.06c 70.02 ± 1.77b

12.98 ± 0.13c 5.57 ± 0.15a 23.59 ± 0.07c 1.21 ± 0.02c 0.21 ± 0.01a 0.31 ± 0.01cd 1.11 ± 0.00bc 71.77 ± 0.52ab

15.57 ± 0.01b 4.38 ± 0.23b 22.78 ± 0.22b 1.25 ± 0.04bc 0.21 ± 0.01a 0.33 ± 0.01ab 1.31 ± 0.10a 74.71 ± 2.08a

20.69 ± 0.27a 3.50 ± 0.29c 24.08 ± 0.04a 1.07 ± 0.05d 0.22 ± 0.00a 0.33 ± 0.01ab 1.31 ± 0.10a 74.47 ± 1.71a

Assays were performed in triplicate. Mean ± SD values in the same column with different superscripts are significantly different (p 6 0.05).A Soymilk concentration is expressed in terms of % w/w (wet basis) total solids content.B Control soymilk as received without ultrafiltration.

Table 5Flow characteristics (Carr index, CI, and Hausner ratio, HR) of spray-dried soymilk powders produced from soymilk (control) and ultrafilteredsoymilk concentrates of different concentrations

Soymilk concentrationA CI (%) HR

5.24 ± 0.03f,B 40 ± 0a 1.67 ± 0.00a

7.04 ± 0.01e 33 ± 2c 1.49 ± 0.05c

9.85 ± 0.01d 32 ± 0c 1.48 ± 0.00c

12.98 ± 0.13c 32 ± 1c 1.47 ± 0.02c

15.57 ± 0.01b 37 ± 2ab 1.60 ± 0.05ab

20.69 ± 0.27a 34 ± 2bc 1.52 ± 0.06bc

Assays were performed in triplicate. Mean ± SD values in the same col-umn with different superscripts are significantly different (p 6 0.05).

A Soymilk concentration is expressed in terms of % w/w (wet basis) totalsolids content.

B Control soymilk as received without ultrafiltration.

Table 6Reconstitution properties (wettability and dispersibility) of spray-driedsoymilk powders produced from soymilk (control) and ultrafilteredsoymilk concentrates of different concentrations

Soymilk concentrationA Wetting time (s) Dispersibility (%)

5.24 ± 0.03f,B 308.0 ± 68.5a 62.3 ± 4.7c

7.04 ± 0.01e 57.4 ± 15.3c 94.0 ± 5.3a

9.85 ± 0.01d 121.0 ± 12.9b 83.6 ± 3.1b

12.98 ± 0.13c 91.6 ± 17.1bc 92.0 ± 2.8ab

15.57 ± 0.01b 112.6 ± 22.8bc 86.9 ± 2.5ab

20.69 ± 0.27a 75.4 ± 10.8bc 92.1 ± 1.9ab

Assays were performed in triplicate. Mean ± SD values in the same col-umn with different superscripts are significantly different (p 6 0.05).

A Soymilk concentration is expressed in terms of % w/w (wet basis) totalsolids content.

B Control soymilk as received without ultrafiltration.

with increasing the total solids contents (Table 4). Thesmall variations of both the mean particle sizes and particlesize distributions (spans = 1.1–1.5) could be responsible forvery small changes in the physical properties of the soymilkpowders. According to Masters (1991), the mean particlesizes that ranged from 20 to 40 lm were classified as veryfine particles.

In terms of handling properties, the spray-dried soymilkpowders had similar flow characteristics and were consid-ered as very cohesive powders by their Hausner ratio(HR) given in Table 5 as classified in Table 2. This is inaccordance with their high Carr index (CI) (Table 5) which

indicated that their flowability was very poor (Table 1).The rationale behind this poor flowability at small particlesizes is due to the large surface area per unit mass of pow-der. There is more contact surface area between powderparticles available for cohesive forces, in particular, andfrictional forces to resist flow (Fitzpatrick, 2005; Fitzpa-trick, Iqbal, Delaney, Twomey, & Keogh, 2004). More-over, the spray-dried soymilks obtained in this studycontained approximately 30% fat (Table 3). This high fatcontent also caused the powder to have very poor flowabil-ity (Fitzpatrick et al., 2004; Perez-Munoz & Flores, 1997b).For the small variation of the particle sizes observed in thisstudy, one could not notice a major change in flowability,however, there should be a noticeable change in flowabilityif the powder size is different by an order of magnitude, forexample, 100 vs 10 lm (Fitzpatrick, 2005). This is truewhen a comparison was made between the flowability ofthe spray-dried powders and the agglomerated powderswhich will be discussed later.

The wettability and dispersibility of the spray-driedpowders produced from the ultrafiltered soymilk concen-trates were significantly (p 6 0.05) higher than those ofthe powder obtained from non-ultrafiltered soymilk butthis effect seemed to be independent of the total solids con-tent of soymilk (Table 6). It can be concluded that theultrafiltration improved the reconstitution properties ofthe obtained soymilk powders. This could be attributedto the partial removal of sugar such as sucrose by the ultra-filtration (Omosaiye et al., 1978). The high sugar contentmade the powder absorbed moisture rapidly which causeda significant reduction in its glass transition temperature.The crystallization of sugar could be initiated and solidcrystal bridge formation between the particles might occur,which could cause caking (Bhandari, Datta, & Howes,1997; Fitzpatrick, 2005; Shittu & Lawal, 2007). This mightcontribute to reduced wettability and dispersibility as theparticles absorb moisture. In this study, it was found thatthe powder obtained from non-ultrafiltered soymilkformed lumps on the surface of water during the wettabilityand dispersibility assays while the other powders showedmuch less tendency to form lumps. In general, it is knownthat water wets very fine powders poorly because of its highsurface tension (Schubert, 1993). A bed of powder remainson the surface of water, with a viscous layer stopping cap-

Fig. 5. Particle size distributions of agglomerated soymilk powdersproduced from spray-dried soymilk (control) powder and aqueoussolution of maltodextrin of different concentrations as a binder.

illary flow in the interparticle porosity (Vu, Galet, Fages, &Oulahna, 2003).

3.5. Proximate analyses of agglomerated soymilk powders

The proximate analyses of the different agglomeratedsoymilk samples are presented in Table 7. As would beexpected, the total carbohydrates content of the agglomer-ated powders increased, whereas protein, fat, and ash con-tents decreased with increasing the binder concentrations.However, it is known that composition of dehydratedsoymilk without additives should contain about 40–50%protein, 21–26% fat, and 20–30% carbohydrate (Wijeratne,1993). Therefore, the compositions of the agglomeratedsoymilk powders obtained in this study have not beenadversely affected by the addition of the binder in the con-centration range tested.

3.6. Physical and morphological characteristics of

agglomerated soymilk powders

Physical properties of the agglomerated soymilk pow-ders as a function of binder concentrations are presentedin Table 8. The spray-dried soymilk (control) from whichthe agglomerates were produced was also included forcomparative purposes. The moisture contents of theagglomerated powders are acceptable for this product(Wijeratne, 1993). In general, the mean particle sizes ofthe soymilk agglomerates were more than an order of mag-nitude larger than that of the control. The size distributionswere generally found to be in good agreement with the log-normal distribution (R2 = 0.950–0.972). The mean particle

Table 7Proximate analyses (% w/w, dry basis) of agglomerated soymilk powders prconcentrations

Binder concentration (%) Protein Fa

ControlA 48.79 ± 1.04ab 280 49.77 ± 0.18a 275 48.02 ± 0.80b 2610 45.78 ± 0.35c 2515 43.21 ± 0.22d 2320 41.91 ± 0.24d 21

Assays were performed in triplicate. Mean ± SD values in the same column wA Control is a spray-dried soymilk powder without agglomeration.

Table 8Physical properties of agglomerated soymilk powders produced from spray-dr

Binder concentration (%) Moisture (%) Particle size (lm) qb

ControlA 4.45 ± 0.11ab 14.54 ± 0.16 0.0 3.70 ± 0.23c 131.20 ± 2.25 0.5 4.48 ± 0.31ab 243.61 ± 1.49 0.10 4.63 ± 0.46ab 259.57 ± 1.47 0.15 4.28 ± 0.05bc 222.76 ± 1.51 0.20 5.02 ± 0.19a 116.95 ± 2.64 0.

Assays were performed in triplicate. Mean ± SD values in the same column wA Control is a spray-dried soymilk powder without agglomeration.

size increased with increasing binder concentrations up to10% and then decreased with further increments. Theagglomeration of soymilk powder also occurred by usingpure water, i.e. 0% binder concentration, indicating thata part of the soymilk powder dissolved during agglomera-tion which eventually acted as a binder.

The particle size distributions of agglomerated soymilkpowders obtained with different binder concentrations arepresented in Fig. 5. The agglomerated powders obtainedwith 0% and 20% binder exhibited high percentage of thefine particles retained on the pan. In contrast, thoseobtained with 5%, 10%, and 15% binder had high percent-age of the coarse particles (>300 lm) with negligibleamounts of the fines. This result indicates that the binder

oduced from spray-dried soymilk (control) powder with different binder

t Ash Carbohydrate

.77 ± 0.36a 4.97 ± 0.01a 17.47 ± 1.40e

.91 ± 0.29a 4.62 ± 0.05b 17.70 ± 0.16e

.17 ± 0.35b 4.39 ± 0.01c 21.42 ± 1.02d

.08 ± 0.48c 4.28 ± 0.06d 24.86 ± 0.83c

.51 ± 0.29d 4.10 ± 0.04e 29.17 ± 0.13b

.85 ± 0.36e 4.29 ± 0.05cd 31.95 ± 0.11a

ith different superscripts are significantly different (p 6 0.05).

ied soymilk (control) powder with different binder concentrations

ulk (g/cm3) qtapped (g/cm3) qparticle (g/cm3) Porosity (%)

21 ± 0.00e 0.35 ± 0.00e 1.25 ± 0.00a 72.00 ± 0.00a

31 ± 0.00b 0.46 ± 0.00b 1.11 ± 0.00b 58.56 ± 0.00c

27 ± 0.01d 0.35 ± 0.00e 1.11 ± 0.00b 68.47 ± 0.00a

31 ± 0.00b 0.38 ± 0.00d 1.04 ± 0.06b 63.26 ± 2.18b

32 ± 0.01a 0.45 ± 0.01c 1.07 ± 0.06b 58.27 ± 2.87c

29 ± 0.00c 0.47 ± 0.00a 1.11 ± 0.00b 57.66 ± 0.00c

ith different superscripts are significantly different (p 6 0.05).

concentrations affected both the mean particle sizes (Table8) and size distributions (Fig. 5) of the resultant agglomer-ated powders which in turn influenced their physical andreconstitution properties.

The variation of the particle sizes with binder concentra-tions could be interpreted that at lower binder concentra-tions, the particles were coated with only a smallproportion of binder which bonded powder particlestogether, while at higher binder concentrations, the binderstarted to spread over the particles in several layers and thecohesion of binder molecules to each other was such thatpowder particles were not bonded to each other by a binderbridge (Planinsek, Pisek, Trojak, & Srcic, 2000; Rohera &Zahir, 1993; Tuske, Regdon, Er}os, Srcic, & Pintye-Hodi,2005). Therefore, when the amount of binder added tothe powder was increased, the friability of the agglomeratesbecame higher. Also, at the equilibrium agglomerationstate, the weight of the agglomerate exceeded the strengthof the interparticle bond. The gravitational force due tothe weight of agglomerates along with forces of shearingamong the particles and between particles and fluidizingchamber, induced by the fluidizing air, caused the agglom-erates to break apart, consequently, reduced the particlesize (Mort, 2005; Rohera & Zahir, 1993; Tan et al.,2006). Turchiuli, Eloualia, et al. (2005) also found thatagglomerates of zein particles obtained with the 14.3% mal-todextrin DE 12 solution were larger and more irregularthan those obtained with the 28.6% solution. This wasdue to the agglomerates obtained with the higher binderconcentration were more friable, while they being subjectedto attrition during the process preventing their growth andleading to regular shaped particles. The effect of viscosityof the binder solutions could be negligible in this studydue to the very low viscosity of the solutions in the concen-tration range used. For 20% binder concentration, theviscosity of the solution was 3.1 mPa s at a shear rate of132 s�1 and 25 �C.

The bulk and tapped densities of the agglomerated pow-ders were significantly higher, whereas their intergranularporosities were lower than those of the spray-dried powder(Table 8). This can be interpreted mainly as the effect ofsize enlargement. As particle size increased, the cohesivityof powder was expected to decrease. A more free-flowingpowder should have a higher bulk density, as the interpar-ticle forces between particles become weaker, thereforepowder should pack in a denser condition (Abdullah &Geldart, 1999). When the above physical properties werecompared among the agglomerates obtained with differentbinder concentrations, the influences of both the mean par-ticle sizes (Table 8) and particle size distributions (Fig. 5)should be accounted for their differences. The agglomeratescontaining the fine particles, i.e. those obtained with 0%and 20% binder, showed significantly higher tapped densi-ties but lower porosities than those without the fines, i.e.those obtained with 5% and 10% binder. This can be attrib-uted to the occupation of the fine particles in the voidsbetween the large particles as pointed out by Abdullah

and Geldart (1999). The agglomerate obtained with 15%binder had high amounts of the small particles (<90 lm)so that its tapped density and porosity were similar to thosecontaining the fine particles. The bulk densities of allagglomerates, however, were not much different, indicatingthat their structural strength was similar under the influ-ence of gravity. The particle density of the agglomeratedpowders was significantly (p 6 0.05) lower than that ofthe spray-dried powder, indicating a more porous structureof the agglomerates. The particle density seemed to be con-stant for all the agglomerates produced with the differentbinder concentrations. This means they had similar intra-granular porosity.

The agglomerated powders obtained in this study had aloose, porous structure and an irregular shape (Fig. 6). Thethicker maltodextrin layers were found on the surface ofagglomerates obtained with the higher binder concentra-tions particularly with the 20% concentration (Fig. 6e).The particle sizes estimated from the visual inspection ofthe micrographs are consistent with those determined bythe sieve analysis.

3.7. Handling and reconstitution properties of agglomerated

soymilk powders

The Carr index (CI) and Hausner ratio (HR) of thespray-dried (control) and agglomerated soymilk powdersobtained with different binder concentrations are presentedin Table 9. Size enlargement by agglomeration seemed toimprove flow characteristics of the powders. The agglomer-ates obtained with 10% binder concentration exhibited agood flowability and low cohesiveness as classified by theCI and HR values given in Tables 1 and 2, respectively.In contrast, the smaller particle sizes and the presence ofthe fine particles obtained with the higher or lower binderconcentrations gave poorer flow characteristics as evi-denced by their higher CI and HR values as compared withthose of the agglomerate obtained with 10% binder.Clearly, the mean particle size and particle size distributionhad a major influence on powder flowability. The coarserpowder in the absence of the fine particles flowed better,as expected. These observations are in agreement with thefindings of other studies (Fuchs et al., 2006; N’Dri-Stemp-fer et al., 2003; Turchiuli, Eloualia, et al., 2005; Turchiuliet al., 2005; Vu et al., 2003).

The effects of binder concentration on particle size,wettability, and dispersibility of the agglomerated soymilkpowders are shown in Fig. 7. It can be seen that the wet-tability markedly increased from the wetting time of 218 sto a satisfactory mean value of 42 s with an increase inbinder concentrations from 0% to 10% and thendecreased sharply to 197 s at 20% binder concentration.This is opposite to the variation in particle sizes with bin-der concentrations. The dispersibility increased from41.5% to 60.8% with increasing binder concentrationsfrom 0% to 10% and then decreased to 52.8% at 20% bin-der concentration. It is known that size enlargement by

Fig. 6. Scanning electron micrographs of agglomerated soymilk powders produced from spray-dried soymilk (control) powder and aqueous solution ofmaltodextrin of different concentrations as a binder: (a) 0%, (b) 5%, (c) 10%, (d) 15%, and (e) 20% w/v at 500� magnification.

agglomeration not only increases the rate of water pene-tration into the space between the agglomerates, but alsothe capillary-driven flow of water into the fine poreswithin the agglomerates and consequently shortens thewetting time (Caric, 2003; Hla & Hogekamp, 1999;Knight, 2001; Schubert, 1993). Hla and Hogekamp

Table 9Flow characteristics (Carr index, CI, and Hausner ratio, HR) ofagglomerated soymilk powders produced from spray-dried soymilk(control) powder with different binder concentrations

Binder concentration (%) CI (%) HR

ControlA 40 ± 0a 1.67 ± 0.00a

0 33 ± 0b 1.48 ± 0.00c

5 22 ± 2d 1.28 ± 0.03e

10 18 ± 0e 1.23 ± 0.00f

15 28 ± 1c 1.38 ± 0.02d

20 38 ± 0a 1.62 ± 0.00b

Assays were performed in triplicate. Mean ± SD values in the same col-umn with different superscripts are significantly different (p 6 0.05).

A Control is a spray-dried soymilk powder without agglomeration.

(1999) pointed out that agglomerates with higher contentof the fine particles showed longer wetting time whichconfirmed our results in case of the agglomerates obtainedwith 0% and 20% binder concentrations.

Fig. 7. Effect of binder concentration on the mean particle size,wettability, and dispersibility of agglomerated soymilk powders.

From the point of view of dispersibility, the particleadhesion within the agglomerates should be strong enoughto avoid abrasion during packaging and transportation, butshould largely dissolve in an aqueous environment, prefer-ably with minimum mechanical energy. In the reconstituteddispersion, the particles should neither float to the surfacenor sediment to the bottom of the container within a certaintime period (Schubert, 1993). The dispersibility of theagglomerates obtained with the 10% binder was the highestamong the other samples probably due to their highest par-ticle size, flowability, and wettability (Vu et al., 2003). Sur-prisingly, the powder agglomerated with water only (0%binder) had a lower dispersibility than the original spray-dried powder (41.5% vs 62.3%). This could be due to theparticle adhesion within the agglomerates being too strongto separate easily on contact with water. However, the dis-persibility of all the agglomerated powders (<61%) wasmuch lower than that of the spray-dried powders obtainedwith the ultrafiltered soymilk concentrates (84–94%; Table6). This could be improved probably by modifying agglom-eration conditions and/or by using other binder materialssuch as maltodextrins with different DE values. In general,the best particle size for rapid dispersion during reconstitu-tion is 150–200 lm (Caric, 2003; Schubert, 1993). Fromthese results, therefore, it can be concluded that the opti-mum binder concentration is 10% w/v maltodextrin.

However, no test was carried out on the agglomerationof the spray-dried powders produced from ultrafilteredsoymilk concentrates, as there were only limited amountsof the powders available, and at the time this experimentwas performed most of the samples were used up for thechemical and physical tests of the primary powders. Never-theless, it is interesting to further investigate if the dispersi-bility of the agglomerates could be improved like the spray-dried powders.

4. Conclusions

It was possible to produce instant soymilk powders in athree-stages process consisting of ultrafiltration, spray dry-ing, and fluidized bed agglomeration. Ultrafiltration con-centrated soymilk up to 20% solids content, leading to anincrease in viscosity, a small change in the proximate com-position, and consequently an increase in particle size ofthe resultant spray-dried powders. However, handlingand reconstitution properties of the powders, i.e. flowabil-ity and wettability, were very poor probably due to theirvery small size (<25 lm). Fluidized bed agglomeration ofthese spray-dried powders was carried out by varying thebinder (maltodextrin) concentrations from 0% to 20% w/v. The optimum concentration was found to be 10% whichgave the agglomerated powder of the highest particle size(260 lm) and in turn the best handling and reconstitutionproperties. Despite the powder wettability being increasedto an acceptable value (42 s), its dispersibility (61%) wasnot sufficiently improved. This is an aspect requiring fur-ther improvement.

Acknowledgements

This study was supported in part by the Thesis Scholar-ships for Master’s degree students, Faculty of GraduateStudies, Mahidol University in the academic year 2004and by the Higher Education Development Project, Sub-project: Graduate Study and Research in Agricultural Bio-technology, Ministry of Education. The authors would liketo thank Green Spot (Thailand) Ltd. (Pathum Thani, Thai-land) for providing the soymilk samples used in theseexperiments.

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